Fix framebuffer mapping for viewport and scissor (flutter#33706)

Author: bdero

impeller/geometry/matrix.h

@@ -27,7 +27,7 @@ namespace impeller {
 ///               * Left-handed coordinate system. Positive rotation is
 ///                 clockwise about axis of rotation.
 ///               * Lower left corner is -1.0, -1.0.
-///               * Upper left corner is  1.0,  1.0.
+///               * Upper right corner is  1.0,  1.0.
 ///               * Visible z-space is from 0.0 to 1.0.
 ///                 * This is NOT the same as OpenGL! Be careful.
 ///               * NDC origin is at (0.0, 0.0, 0.5).

impeller/renderer/backend/gles/render_pass_gles.cc

@@ -281,14 +281,20 @@ struct RenderPassData {
       gl.Disable(GL_DEPTH_TEST);
     }
 
+    // Both the viewport and scissor are specified in framebuffer coordinates.
+    // Impeller's framebuffer coordinate system is top left origin, but OpenGL's
+    // is bottom left origin, so we convert the coordinates here.
+    auto target_size = pass_data.color_attachment->GetSize();
+
     //--------------------------------------------------------------------------
     /// Setup the viewport.
     ///
     const auto& viewport = command.viewport.value_or(pass_data.viewport);
-    gl.Viewport(viewport.rect.origin.x,    // x
-                viewport.rect.origin.y,    // y
-                viewport.rect.size.width,  // width
-                viewport.rect.size.height  // height
+    gl.Viewport(viewport.rect.origin.x,  // x
+                target_size.height - viewport.rect.origin.y -
+                    viewport.rect.size.height,  // y
+                viewport.rect.size.width,       // width
+                viewport.rect.size.height       // height
     );
     if (pass_data.depth_attachment) {
       gl.DepthRangef(viewport.depth_range.z_near, viewport.depth_range.z_far);
@@ -300,10 +306,11 @@ struct RenderPassData {
     if (command.scissor.has_value()) {
       const auto& scissor = command.scissor.value();
       gl.Enable(GL_SCISSOR_TEST);
-      gl.Scissor(scissor.origin.x,    // x
-                 scissor.origin.y,    // y
-                 scissor.size.width,  // width
-                 scissor.size.width   // height
+      gl.Scissor(
+          scissor.origin.x,                                             // x
+          target_size.height - scissor.origin.y - scissor.size.height,  // y
+          scissor.size.width,                                           // width
+          scissor.size.height  // height
       );
     } else {
       gl.Disable(GL_SCISSOR_TEST);